Fifteen people have been killed when gunmen burst into a church in Kenya.

The attackers killed two policemen guarding the church, snatched their rifles and then opened fire on the congregation. Forty people were wounded, said security officials.

A grenade attack against a second church in the city of Garissa wounded three people.

Two gunmen entered the simple wooden church at around 10:15 am while two others waited outside, police said. When the congregation fled the attack inside, they ran straight into another hail of bullets from gunmen outside. At least one grenade was detonated in the attack.

"We were deep in prayers preparing to give our offerings," said a visibly shaken David Mwange, a churchgoer. "We first had a loud bang from outside which we mistook to be coming from the rooftops. We then had gun shots which made us to lie down. Within no time we had gunshots all over. Everybody was shouting and wailing in pain."

Garissa Mayor Ismail Garat called the church assault "evil." He said: "We are not used to witnessing such kinds of acts in our country, where people are just shot in broad daylight. We really want to know who the heartless people who did this are."

Police suspect the attacks were carried out by al-Shabab, the most dangerous militant group in Somalia. The police were guarding the church because of the increasingly dangerous security situation near the border with Somalia and because Somalia's Islamist militants have made Christian churches a common target.

The Vatican spokesman condemned the "vile" and "disgraceful" attacks and said they showed the necessity of defending the rights of Christians to celebrate their faith and "oppose irresponsible acts that fuel hatred among religions."

Kenya sent troops into Somalia last October to hunt al-Shabab fighters. The militants, who are allied with al Qaida, have threatened repeatedly to carry out revenge attacks for Kenya's push into Somalia.